Police have located the booby-trapped car of the suspect, the girl's step-uncle, and were overnight defusing a bomb in the vehicle, Der Tagesspiegel reported citing police sources.
Explosives specialists descended on the neighbourhood in the eastern district of Friedrichshain after the red BMW was found late Friday, and more than 100 residents were evacuated while the operation was underway.
A complicated device made up of "wires and pipes" was inside the vehicle, police said.
The suspect, whose name was given by the ,i>Berliner Zeitung> newspaper as 32-year-old Peter John, allegedly placed a bomb in a letterbox, which exploded and critically injured the girl on Wednesday when she opened it at her family's home in Rudow.
Doctors have been frantically trying to save the girl's life, and they said there's now a good chance they will be able to save her arm. But the girl, Charlyn, remained in critical condition at a Berlin area hospital.
"Dangerous toxins could spread through her body," Andreas Eisenschenk, the lead surgeon on the operation, told the Tagesspiegel.
The suspect also allegedly put a second bomb on the roof of the car of the girl's father, but that did not go off. He has been on the run since the incident, and police have sealed off his apartment.
A neighbour reported that the suspect never made an unpleasant impression and is supposedly “a calm and friendly man,” according to the paper.
He had always felt mistreated when compared to the standard of living of the rest of the family and a dispute has been ongoing for years.
The father reportedly revealed during police questioning on Thursday that he suspected someone in his extended family might want to harm his immediate family, but refused to comment further.